1. Field of the Invention
The present invention generally relates to an exposure apparatus and a device manufacturing method using the exposure apparatus. More particularly, though not exclusively, the present invention relates to an exposure apparatus utilizing, as exposure light, light in an extreme ultraviolet range (EUV light) with a wavelength of between 5 nm and 20 nm, and a method of manufacturing a device, such as a semiconductor device or a liquid crystal display device, by using the exposure apparatus.
2. Description of the Related Art
To meet demands for finer device patterns, the wavelength of light used as exposure light in an exposure apparatus has become shorter and shorter.
Recently, various proposals have been made with regards to an exposure apparatus using, as exposure light, the EUV light with a wavelength of 5 nm to 20 nm (hereinafter referred to as an “EUV exposure apparatus”) (see, e.g., Japanese Patent Laid-Open No. 2003-309057).
In the EUV exposure apparatus, because there are no materials having refractive indices practically adapted for the EUV light, a transmission optical system (dioptric system) cannot be employed as an illumination optical system for illuminating a mask (reticle) with light from an exposure light source, and a reflection optical system using a mirror coated with a reflection multilayered film is employed as the illumination optical system.
For the same reason, a reflection optical system using a plurality of mirrors is employed as a projection optical system for projecting a mask pattern onto a substrate, such as a wafer or a glass plate.
Each mirror has a multilayered reflection film made up of several tens of layers each formed of a Mo/Si pair, and it reflects the EUV light with a equivalently high refractive index.
FIG. 2 illustrates an EUV exposure apparatus discussed in Japanese Patent Laid-Open No. 2003-309057. In FIG. 2, an exposure light source unit 1 for emitting EUV light includes, e.g., a discharge-produced plasma light source in which Xe gas or Sn vapor is brought into a plasma state with a discharge, thus generating the EUV light, or a laser-produced plasma light source in which a high-power pulse laser beam is condensed and irradiated to Xe or Sn, thus producing plasma.
An illumination optical system 3 includes a plurality of mirrors. A convex mirror 11 and a concave mirror 12 constitute a parallel conversion unit for receiving a beam of EUV light emitted from the light source unit 1 and converting it to a substantially parallel light beam. A reflection integrator 4 has a plurality of cylindrical reflection surfaces. Above the reflection surfaces of the integrator 4, an aperture stop 15 is disposed with its opening surface positioned substantially perpendicular to the integrator front face. The aperture stop 15 specifies a distribution shape of an effective light source and also specifies an angle distribution of the light that illuminates respective points on a reflection mask (reticle) 6, i.e., a surface to be illuminated.
A convex mirror 13, a concave mirror 14, and a plane mirror (folding mirror) 2 constitute a condenser unit for condensing the light beam from the integrator 4 into a circular arc shape. The plane mirror 2 reflects the image-side light beam in the condenser unit upwards so as to enter the reflection mask 6 at a predetermined angle.
The reflection mask 6 is held by a mask chuck 50 provided on a mask stage 5. A projection optical system 7 is a coaxial optical system constituted by a plurality of mirrors coated with multilayered films. The projection optical system 7 is designed such that the object side is non-telecentric and the image side is telecentric.
A wafer 8 coated with a photosensitive material is held by a wafer chuck 90 provided on a wafer stage 9. A vacuum chamber 20 keeps the interior of the entire exposure apparatus in a vacuum state.
FIG. 3 is a schematic perspective view of the reflection integrator 4, which can have a plurality of convex cylindrical surfaces, in a state where a substantially parallel EUV light beam 10 enters the integrator 4. The substantially parallel EUV light beam 10 enters the integrator 4 in a direction as shown.
As illustrated in FIG. 3, when the substantially parallel EUV light beam 10 enters the integrator 4, which can have the plurality of convex cylindrical surfaces, a plurality of linear secondary light sources are formed near the integrator front face, and respective EUV light beams radiated from the plurality of secondary light sources have an angle distribution in the form of a conical surface. Those EUV light beams are reflected by a mirror, which can have a focal point matched with the positions of the secondary light sources, so as to illuminate the reflection mask 6. Thus, the light beams from the plurality of secondary light sources are superimposed with one another on the reflection mask 6 such that the mask can be illuminated in the circular arc shape.
Various proposals have also been made regarding methods for measuring optical performance of the projection optical system in the EUV exposure apparatus (see, e.g., Japanese Patent Laid-Open No. 2000-97622, Japanese Patent Laid-Open No. 2003-302205, U.S. Patent Application No. 2002/0001088, and U.S. Pat. No. 5,835,217).
Known interferometers for measuring a wavefront aberration of the projection optical system in the known EUV exposure apparatus will be described below.
Interferometers for measuring a wavefront aberration of the projection optical system in the EUV exposure apparatus include, for example, a shearing interferometer, a point diffraction interferometer (PDI), and a line diffraction interferometer (LDI), each of which employs a combination of SOR (Synchrotron Orbit Radiation) and UNDULATOR as a light source. FIG. 7 is a conceptual view of an interferometer for measuring a wavefront aberration of an optical system to be tested. EUV light 21 generated by the combination of SOR and UNDULATOR is introduced to a vacuum chamber 23 through an illumination optical system 22. After being reflected by a folding mirror 24, the EUV light 21 passes an optical system 31 to be tested and is detected by a detector 30, e.g., a CCD (Charge Coupled Device). A first mask 25, which can have a pinhole or a window, a first diffraction grating 26, a second diffraction grating 27, and a second mask 28, which can have a pinhole or a window are disposed, as required, in an optical path as shown. The vacuum chamber 23 is supported by a vibration isolation base 29. In the interferometer shown in FIG. 7, a projection optical system using two mirrors is disposed as the tested optical system 31.
The interferometer of FIG. 7 is able to evaluate the tested optical system 31 in accordance with plural types of interferometry. FIGS. 8A and 8B are conceptual views of two typical types of interferometry.
FIG. 8A shows a PDI (Point Diffraction Interferometer), and FIG. 8B shows an LSI (Lateral Shearing Interferometer). In these figures, the same parts as those in FIG. 7 are denoted by the same reference numerals.
The PDI will be first described with reference to FIG. 8A. The light 21 from the light source is condensed onto the first mask 25 disposed in an object plane of the tested optical system 31. The first mask 25 has a pinhole formed therein with a diameter of not larger than a diffraction limit (i.e., not larger than λ/2NA where NA is the numerical aperture of the tested optical system 31 and λ is the wavelength of the light 21). After passing the pinhole of the first mask 25, the light 21 is shaped to a substantially ideal spherical wave. The light having passed the first mask 25 is diffracted by the first diffraction grating 26 to enter the tested optical system 31, and is condensed onto the second mask 28 disposed on an image plane of the tested optical system 31. The second mask 28 has a pinhole formed therein with a diameter of not larger than the diffraction limit and a window formed therein with a size sufficiently larger than the diffraction limit. Because the light condensed onto the second mask 28 has been diffracted by the first diffraction grating 26, plural beams of light are condensed to respective positions on the second mask 28 depending on the order of diffraction. The second mask 28 and the first diffraction grating 26 are aligned with each other such that the light of 0th order passes through the pinhole and the light of +1 or −1 order passes through the window.
The beams of light of the other orders are all shielded by a light shielding portion of the second mask 28.
The light diffracted by the pinhole of the second mask 28 can be regarded as a substantially ideal spherical wave and is deprived of wavefront aberration information of the tested optical system 31. On the other hand, the light having passed through the window of the second mask 28 contains the wavefront aberration information of the tested optical system 31.
An interference fringe (interference pattern) produced by those two beams of light is observed by the CCD 30 serving as the detector. A method of measuring the wavefront aberration of the tested optical system from the interference fringe can be practiced as the so-called electronic moiré method using one pattern of interference fringe superimposed with a tilt fringe, or the so-called Fourier transform method. Another practicable option is to employ the so-called fringe scan method in which plural patterns of interference fringes are obtained by the CCD 30 while scanning the first diffraction grating 26 in a direction perpendicular to an optical axis.
The LDI (Line Diffraction Interferometer) includes employing, instead of the pinholes, slits as the first mask 25 and the second mask 28 in the arrangement of FIG. 8A. In the case of the LDI, since the amounts of light passing the first mask 25 and the second mask 28 are increased, the measurement can be easily performed even when the light source has a low luminance.
The LSI will be described with reference to FIG. 8B. The light 21 from the light source illuminates the first mask 25. The first mask 25 has a pinhole formed therein with a diameter of not larger than the diffraction limit.
The light diffracted by the pinhole of the first mask 25 passes the tested optical system 31 and then enters the second diffraction grating 27. The second diffraction grating 27 diffracts the light outgoing from the tested optical system 31 to form a plurality of light-condensed points on the second mask 28. The second mask 28 has two order-selection windows formed therein with a size of sufficiently larger than the diffraction limit such that only beams of light of ±1 order among the light diffracted by the second diffraction grating 27 are facilitated to pass the windows. The beams of light having passed the second mask 28 interfere with each other, and an interference fringe is observed by the CCD 30. Phase information can be obtained from the interference fringe by using the electronic moiré method or the fringe scan method. The thus-obtained phase information represents a differential value of the wavefront aberration of the projection optical system (tested optical system 31) in one direction (e.g., a differential value in the x-direction). After rotating the second diffraction grating 27 and the second mask 28 through 90° about the optical axis, an interference fringe is similarly observed by the CCD 30 and phase information is obtained in the rotated state. The thus-obtained phase information represents a differential value of the wavefront aberration of the tested optical system 31 in the y-direction. The information regarding the wavefront aberration of the tested optical system 31 can be obtained from those differential values of the phase information in the two x- and y-directions.
While the first mask 25, which can have the pinhole, is employed in the above description, a slit 25′ or 25″ extending in parallel to the second diffraction grating 27 in each rotated state thereof can be formed in the first mask.
Alternatively, in the interferometer of FIG. 8B, the second diffraction grating 27 and the second mask 28 can be replaced respectively with a two-dimensional grating 27a, e.g., a cross-grating, and a mask 28a having four order-selection windows with a size of sufficiently larger than the diffraction limit. This arrangement enables the information regarding the wavefront aberration of the tested optical system 31 in the two x- and y-directions to be obtained contemporaneously.
The projection optical system used in the EUV exposure apparatus is highly sensitive to position accuracy and heat-induced deformations of the mirrors forming the projection optical system. It is therefore required to ensure the performance of the projection optical system in the exposure apparatus through, e.g., feedback control of measuring the wavefront aberration of the projection optical system and adjusting the mirror positions during an interval between the exposure operations.
Also, impurities can attach to or cause chemical changes of the Mo/Si multilayered film coated on the mirror, and can bring about phase changes, due to the so-called contamination. This necessitates measuring the optical performance of the projection optical system in the exposure apparatus at the exposure wavelength used in practice.
In connection with such measurement, the inventor has found the following. When trying to mount, in the EUV exposure apparatus, a measuring unit (e.g., an interferometer) for measuring optical characteristics of the projection optical system therein, one conceivable solution is to arrange, as the first mask, a mask, which can have a reflection area in the form of a pinhole or a slit instead of the usually employed reflection mask. However, such an arrangement cannot provide the amount of light sufficient to measure the optical characteristics of the projection optical system.
The reason is that, in the illumination optical system of the EUV exposure apparatus, because secondary light sources are formed using an integrator and beams of light from the secondary light sources are superimposed with one another on an illuminated plane to perform uniform Koehler illumination on a reflection mask, the illuminance on the illuminated plane is generally reduced.
Accordingly, an exposure apparatus is demanded which enables illumination to be performed in a manner suitable for any type of mask regardless of which one of plural types of masks (e.g., a transmission mask, a reflection mask, and a plurality of reflection masks having different pattern areas from one another) is disposed in the object plane of the projection optical system in the exposure apparatus.